When it comes to LED failure, people first think of the dead light that appears under normal current drive, or just emits faint light. In fact, this is the most serious type of failure, called disaster failure. Conversely, if the LED product is in normal use, some key parameter characteristics deviate from acceptable limits, such as permanent light output attenuation, color temperature drift, color rendering index drop, etc., which we call parameter failure.
When it comes to LED failure, people first think of the dead light that appears under normal current drive, or just emits faint light. In fact, this is the most serious type of failure, called disaster failure. Conversely, if the LED product is in normal use, some key parameter characteristics deviate from acceptable limits, such as permanent light output attenuation, color temperature drift, color rendering index drop, etc., which we call parameter failure. Considering the bare chip (ie, the epitaxial grain) alone, the probability of failure of the LED product parameter is very low, because it belongs to a very stable solid compound, which is used under standard conditions and is not easily damaged, but is in general application. The environment does not react chemically and therefore has a long life. However, in order to illuminate the chip, it must be adhered to a specific stage (ie, a support or a substrate) and connected to the positive and negative electrodes of the die by a metal wire or a solder material, and then mixed with a polymer material and a luminescent material. Covering the entire stage, this is the so-called packaging process. After the process of the LED lamp beads, the package material coated on the chip is extremely vulnerable to damage. Therefore, the failure of various LED parameters is attributed to the destruction and deterioration of the packaging material. .
Most of the parameter failure process is a gradual process, and can not be immediately detected at the beginning, it is a hidden danger, called recessive failure. After a period of time, important materials were completely destroyed and eventually became disasters. The vulcanization phenomenon belongs to this recessive failure.
At present, the phenomenon of LED blackening at the application end is due to the poor performance of vulcanization of the silver plating layer of the stent. From the statistics of all vulcanization cases, the defect mainly occurs in the TP white light series products. The occurrence of vulcanization phenomenon is not necessarily related to which LED chip is selected. Most of the vulcanization failure mainly occurs in the LED product of the silica gel process (-S) package, and the probability of vulcanization in the S1 silicone resin process is relatively low.
The reason for the blackening of the bottom of the stent is that the external sulfur ion S2 invades the Ag layer of the internal support of the LED lamp with water molecules in the air as a carrier, resulting in the formation of sulfide under certain conditions. From the various adverse cases that occur, the intensity and speed of vulcanization of the stent silver layer is directly related to the sulfur content, temperature and time. The sulfurized substances or particles were analyzed by EDS and found that in addition to the large amount of Ag (silver) signals, the secondary S (sulfur) signal is also very obvious, so it can be determined that the blackening problem is due to the chemical reaction caused by the reaction between Ag1+ and s2Q. Silver has a strong affinity for sulfur, and can directly synthesize Ag2S with sulfur when heated.
The vulcanization of LEDs is due to the infiltration of sulfur (S2ˉ) elements into the interior of the LED holder. Under certain conditions of temperature and humidity (heat promotes molecular motion), the chemical reaction of -2 valence with +1 valence silver The process of black Ag2S. Because of structural and process reasons. Sulfuration reaction equation: Ag2S "="2Ag(1+) + S(2-)
LED vulcanization hazard:The early appearance of the LED after vulcanization showed blackening of the functional area of ​​the stent, a serious decrease in luminous flux, and a significant shift in color temperature. Since the conductivity of silver sulfide increases rapidly with increasing temperature, some of the leakage may occur during the use of the vulcanized LED, especially if the packaged wafer is a small size or a PN junction near the bottom. As the silver layer of the stent is aggravated by the degree of vulcanization, the LED will eventually fail completely and a dead light problem will occur.
From the vulcanization failure analysis process and reliability test, summarize some anti-vulcanization measures:
1. LED application products should reduce or replace materials containing sulfur, such as rubber products, sulfur soap and so on. Avoid storage in the same space with sulfur or oxidizing substances.
2. Use quality-guaranteed PCB board, solder, and other auxiliary materials to avoid residual sulfur on the PCB.
3. After the PCB reflow soldering is completed, the surface residue is removed or reduced by cleaning the solder joints. The cleaning agent avoids the use of acidic sulfur-containing adhesives and solvents.
4. The packaging process uses modified silicone resin with small molecular gap, good air tightness and high resistance to vulcanization.
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